>  but when I try
> to boot, I get something like this at the bottom of the screen:
> 
> F1: windows
> F2: netbsd
> 3
> 
> Of course the F1 and F2 lines are printed by the bootloader.
> The 3 gets printed when I either select F2, or wait for the
> bootloader timeout.  Any ideas on how to debug this problem
> further?

RTFM. mbr(8) sayes error 3 is "No operating system.  The bootsector
was loaded successfully, but it was not valid (i.e.  the magic number
check failed)."

What you have (probably) forgotten to do is run /usr/mdec/installboot
alternatively:
- your root filesystem isn't at the start of the BIOS partition
- your root filesystem is in a part of the disk that can't be
  read using bios commands - 8Gb for old bioses.

	David

-- 
David Laight: david@l8s.co.uk